Inclusive and differential measurement of top quark cross sections in association with a Z boson

A measurement is presented of the inclusive and differential cross sections for top quark production in association with a Z boson, in pairs ($\mathrm{t\bar{t}Z}$) or with a single top quark ($\mathrm{tZq}$ and $\mathrm{tWZ}$). The data were recorded in pp collisions at a center-of-mass energy of $13~\mathrm{TeV}$, corresponding to an integrated luminosity of $138~\mathrm{fb}^{-1}$. Events with exactly three leptons, electrons or muons, are selected. A deep neural network is trained to separate the signal processes and the backgrounds. The $\mathrm{t\bar{t}Z}$ and $\mathrm{tWZ}$ processes are measured together due to their similar experimental signature and significant interference beyond leading order. A combined profile likelihood approach is used to unfold the differential cross sections, to account for systematic uncertainties, and to determine the correlations between the two signal categories in one global fit. The inclusive cross sections for a dilepton invariant mass within $70$ and $110~\mathrm{GeV}$ are measured to be $1.14\pm0.07~\mathrm{pb}$ for the sum of $\mathrm{t\bar{t}Z}$ and $\mathrm{tWZ}$, and $0.81\pm 0.10~\mathrm{pb}$ for $\mathrm{tZq}$. While good agreement of the data with the standard model prediction is found for the $\mathrm{tZq}$ process, the measured inclusive cross section for $\mathrm{t\bar{t}Z}+\mathrm{tWZ}$ has a ratio to the central value of the prediction of $1.17 \pm 0.07$.